Birds generally don't get electrocuted on power lines because they are perched on a single wire, which means both feet are at the same electrical potential.

 Birds generally don't get electrocuted on power lines because they are perched on a single wire, which means both feet are at the same electrical potential. Electricity flows from areas of high potential to low potential, and because there is no potential difference between the bird's feet, the current doesn't pass through its body. However, birds can be electrocuted if they touch two wires simultaneously or if they touch a wire and a grounded object, like the pole supporting the wire. 

Here's a more detailed explanation:

Potential Difference:

Electricity flows when there's a difference in electrical potential between two points. When a bird is perched on a single power line, both feet are at the same potential, so there's no path for electricity to flow through the bird. 

No Circuit:

For electricity to flow, a circuit needs to be completed. If a bird is only touching one wire, there's no circuit, and therefore no current flows. 

Grounding:

If a bird touches a grounded object (like the pole or the ground) while also touching a power line, it creates a path for electricity to flow from the wire, through the bird, and to the ground, causing electrocution. 

Simultaneous Contact:

Similarly, if a bird touches two power lines simultaneously, it creates a circuit between the wires, and electricity will flow through the bird.